Glaucoma is an optical neuropathy associated with increased intraocular pressure. The mechanism of the disease is not fully understood. However, the most effective therapy appears to be reducing the intraocular pressure, for example using medication or implants. Further damage to the optic nerve is thus prevented or reduced.
One procedure that has been suggested is non-penetrating trabeculectomy, in which a portion of the sclera overlying the Schlemm's canal is removed, allowing aqueous humor to leave the eye. It is desirable to remove only part of the thickness of the sclera, preventing penetration into the eye. However, this procedure is difficult to perform with a knife. Typically, the effect of the procedure can only be gauged after a while, since intra-ocular pressure is only measured after the procedure is completed. As the pressure of the knife causes trauma to the eye, the pressure is not usually measured until the eye has somewhat recovered, such as the next day. In laser based procedures, such as SLT and ALT, pressure is sometimes measured after the procedure is completed, to ensure that the intra-ocular pressure did not suddenly rise.
U.S. Pat. No. 5,370,641 to O'Donnell, the disclosure of which is incorporated herein by reference, describes using an Excimer laser or an Erbium laser to ablate the sclera overlying the Schlemm's canal and the trabecular meshwork thereby forming a porous membrane. The laser spot size and treatment area are not described. This patent states that when a sufficient amount of the corneoscleral bed is removed, aqueous humor comes through the remaining ultra-thin Schlemm's canal and trabecular meshwork and the energy of the laser is absorbed by the out-flowing humor, creating a self-regulating end point.
However, even though many years have passed since this patent was issued, the method taught in the patent has not found wide-spread use, in spite of a great need in the art of treating Glaucoma, a disease for which there is no completely satisfactory treatment. One possible reason is that the '641 patent uses lasers that remove very thin (micron sized) layers of material. Further, once even a weak percolation starts, the laser is only effective to remove the percolation, not further tissue, while at the same time possibly causing thermal damage to the underlying tissue. This thermal damage may be a cause of later scarring.